1. Field of the Invention
This invention relates to additives for molds for use in casting of steel. More particularly, this invention relates to additives for continuous casting which are added to the surface of a molten steel in a mold in order to prevent surface defects on billets and final products derived therefrom in continuous casting; the additives for continuous casting (hereinafter referred to simply as additives) permit the casting of economical, high quality billets, and final products having a high surface clean ability.
2. Description of the Prior Art
Generally, the characteristics required of this type of additive (continuous casting powder) include (1) that it has appropriate melting points, (2) that it has appropriate viscosity characteristics and slag fluidity, and the slag skin is not broken, (3) that it has appropriate basicities, and (4) that free CaO is not present because it promotes the formation of calcium aluminate and is thus undesirable.
Most known additives are multi-component mixtures of appropriate combinations of metal oxides such as SiO.sub.2, CaO, Al.sub.2 O.sub.3, Na.sub.2 O, K.sub.2 O, B.sub.2 O.sub.3 and Li.sub.2 O and with metal fluorides such as CaF.sub.2, NaF, LiF and AlF.sub.3 contained therein. By adding these mixtures to the surface of a molten steel, they melt and serve various purposes, e.g., preventing the surface oxidation of the in-mold molten steel, insulating heat by blocking its radiation, absorbing scums floating on the surface of the molten steel, and, in continuous casting, imparting lubricating action between the steel and the mold.
These known multi-component mixtures usually comprise, for example, fly ash, glass powder, clay powder (perlite, diatomaceous earth, bentonite, etc.), portland cement, etc. as base materials added with flux components such as fluorides, borides, alkali carbonates, etc., carbon for adjusting the melting speed, and their compositions are roughly 30 to 50% SiO.sub.2, 2 to 15% Al.sub.2 O.sub.3, 20 to 50% CaO, 5 to 15% alkali oxides, and 2 to 10% of carbon materials, all by weight.
However, such prior art additives suffer from problems, such as (1) that the composition of base materials is complicated and the melting characteristics will vary widely according to changes in the kind of component in the composition or in the amounts thereof added, (2) that there occur surface defects of final products due to unevitable incorporation of calcium aluminate in base material components, and so forth.
Furthermore, with the prior art additives in which flux-forming components comprise several kinds of multicomponent mixtures, since when they are added to the surface of a molten steel, they undergo a two-stage reaction, i.e., initially sintering takes place, and subsequently the sintered product melts, the thermal change on the steel surface is great, and even in static casting such as top pouring casting, bottom pouring casting, etc., when the pouring speed is increased, the balance between sintering and melting may be lost, generating a sintered ingot, and thus sometimes blocking the operation. Still more, for continuous casting, in which an ingot is formed under a dynamic equilibrium condition, this equilibrium is extremely delicate, and it is therefore apt to cause problems. In continuous casting, in which the additive added flows between the mold and the solidified shell, and is withdrawn together with the steel, the height of the molten layer from the steel is determined mainly by the balance of the casting speed and the melting speed of the additive. If the melting speed is too rapid, all of the additive will melt, thus deteriorating the heat insulating effect, while if it is too slow, the molten layer will disappear, and it will intervene as a powder between the steel and the mold, thus causing defects on the steel surface. In an extreme case, the so-called break out phenomenon may occur; that is, the nonsolidified molten steel may flow out, which can sometimes render the operation impossible. Thus, especially for ingot formation in continuous casting, the effect of the additive is extremely important, and is very delicate with respect to the necessity of maintaining the dynamic equilibrium condition. For the reason the additive should not only be of a specified chemical composition, but also the chemical and physical properties of the additive must comply with extremely severe requirements.
On the other hand, as the demand for improving productivity of steel has become stronger, so-called high speed continuous casting, which is conducted at even higher speeds, has been attempted, and accordingly there has been an increasing demand for additives suitable for such process.
For example, while the drawing speed in ordinary continuous casting is from about 1 to 1.4 m/min., there is an increasing inclination toward the so-called high speed continuous casting operations conducted at about 1.8 m/min. or higher.
However, in order to increase producitivity by such continuous casting, the characteristics of the prior art additives are inadequate. The properties that are required for improved performance include lower viscosity, a higher film strength of the molten glass, and a lower surface tension (to have a greater ability to wet the molten steel). In complying with these requirements, various measures have been proposed, such as increasing amounts of Na, K, etc. added, increasing F content, etc. from the compositional aspect, and, for example, the use of pre-calcined starting materials in order to prevent retardation in melting due to sintering on the steel surface, heating starting materials to vitrify them, and so forth.
However, if the amount of Na, K, or the like added is increased, the molten ingot loses its vitreous properties and becomes easily crystallized, and the strength of the glass film is lowered, and hence becomes apt to generate surface defects of the steel. Furthermore, when the increased amounts of Na or K are added as the fluoride or carbonate, the amount of SiF.sub.4 and CO.sub.2 gases generate increases, due to its reaction with the silicate content and an autodecomposition reaction.
On the other hand, the proposals on the sintering treatment and melting treatment of additives not only still have room for improvement because of merits and demerits in controllability of the melting speed and heat insulating properties, but also because it is disadvantageous to employ such a production process for the limited production of each of multi-items. For additive manufacturers such individual sintering and melting treatments are not industrial, thus resulting in low productivity.
As a result of our intensive study on additives in view of the above situations, we have discovered base materials for additives which satisfy these requirements, and thus accomplished the present invention.